Rotor Steeple Indications and Diaphragm Repairs
Mechanical Dynamics & Analysis (MD&A) experts recently completed steampath inspection and repair services for a customer during a major overhaul. The Alstom® steam turbine consisted of an HP/IP turbine (stages 1-15) and a single flow LP turbine (stages 16-21).
A detailed incoming inspection was performed for the HP/IP and LP rotors and found relevant indications in the LP rotor steeples at L-1 (stage 20) through magnetic particle inspection, in addition to minor damages identified on the HP/IP rotor.
Finding indications in the last-stage LP rotor steeples (also known as fir-tree grooves) is a common issue. Basically, the steeple indications are caused by corrosion fatigue (when a material degrades due to the combination of corrosion and cyclic stresses).
Repair of the LP rotor included removal of the L-1 steeple indications by hand tooling, followed by blending and polishing. There were 3 steeple indications which were noticeably deeper than the rest. The approximate depth of these deeper indications was 3 mm (0.12”) located on the convex side of the steeple.
A finite element analysis was performed to verify the structural integrity of the steeples after grinding. Based on the analysis, the L-1 steeple repair was found to be acceptable. After all rotor repairs were complete, the rotors were balanced, and the journals were strapped lapped prior to leaving the workshop.
Incoming inspections were also completed on the HP/IP and LP diaphragms. The HP/IP diaphragms arrived at the workshop with the lower half appendages removed for reverse engineering. Minor damages were noted on the LP diaphragms including horizontal joint erosion and foreign object damage to the partition trailing edges on all stages, as well as damage to the steam seal face on stages 17-21.
Repair work to the HP/IP diaphragms included installing new appendage rings with honeycomb seals, which were supplied by MD&A Parts. To ensure proper fitting, the appendage and bore packing fit diameters were machined round prior to installation of the new appendages. Additionally, the dowel pins were removed, and the horizontal joint was skim cut. Dowel pin holes were then enlarged, and oversized dowel pins were installed.
New bore packing (with honeycomb seals) for stages 2-9 and 11-15 were also supplied by MD&A Parts. The bore packing was test fitted after appendage installation.
Minor repairs were performed on the LP diaphragms including horizontal joint weld repairs and removal of broken studs. After all diaphragm repairs were complete, new horizontal joint hardware was supplied by MD&A Parts and test-fitted before the diaphragms left the workshop.
Through MD&A’s state-of-the-art repair facility and quick access to replacement parts, an extensive inspection and necessary repairs were completed by our experts in a timely manner that allowed the unit to be quickly returned to the site.
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